I wondered the same thing myself. I got my cert over the weekend at Dutch Spring and we hit 2 themoclines, surface temp 81, 34' temp was 75, 57' temp was 62-63.
I was planning to hit the caribbean after getting certified and asked would it be better to get my AOW first. All answers to the question were a definite yes! And after thinking about I admit their right for various reasons.
1- To get more dives under my belt and felt more comfortable at depths beyond 40
2 - Alot of good wrecks, reefs sights in general are at depths greater than 40 (depending where you go of course). Aruba, Keys and Cozumel seem like the exceptions.
3 - I dont really want to do my AOW with someone I haven't been diving with as I love the way my LDS works.
4 - Want to do Nitrox before I go also.
Congrats on your cert though, getting it just makes you want to dive more, atleast for me!!